In French patent application No. 89 13488 of Oct. 16, 1989, and No. 92 06733 of Jun. 3, 1992, the Applicant has described a three-phase line for conveying electrical power, using compressed nitrogen insulation and designed to be placed on or buried in the ground. That electricity line comprises a steel outer envelope having three conductors disposed therein, each corresponding to one of the phases, each conductor being surrounded by a respective envelope that protects it from dust. The resultant of the magnetic field due to the flow of balanced current in the three conductors is substantially zero at a short distance (several meters) such that such a line does not present any danger for people living nearby. However, at the short distance between the steel tube and the conductors themselves, the magnetic field remains large, and therefore causes eddy currents to appear in the steel tube which in turn generate Joule losses that give rise to heating.
It is known, in particular from document WO-A-79 00607, to reduce the above-mentioned losses by placing a screen of non-magnetic metal, e.g. aluminium, between the conductors and the steel envelope.